CND problem. Help please

I am facing a problem with CDN. CDN isn’t work properly. My blog is situated in sub-directory (ex: https://www.example.com/blog) I was trying to use sub-domain CDN. For this, I have created a sub-domain (cdn.example.com) & put “wp-content” & “wp-include” folders in cdn.example.com/blog sub-directory (because my blog is situated in sub- directory). Then in “CDN” tab of WP Super Cache, I have put “https://cdn.example.com/blog” in “off-site url” field. Then I have enabled cdn option & pressed “save changes”

Now, here is a deal. My blog has 2 activated plugins, NextGEN & WP Super Cache. After activating CDN, blog has loaded without any images ?? Then I have seen the page’s source and copied a image link and gone to this link manually by browser (this image was a gallery image of NextGEN). At first, page says “page not found”, but when I have reloaded the page again, the image has loaded successfully. What happended in first time loading? I have no idea where is the actual problem. Please help.